The present invention relates generally to variable speed drives. More particularly, the present invention relates to the reduction of harmonics produced by a variable speed drive through a modulation scheme.
This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Variable speed drives are used to drive a variety of electric motors. One exemplary application is to drive a submersible motor used in an electric submersible pumping system. A typical system comprises a pump, an electric motor and a motor protector deployed in a wellbore. The submersible motor can be used to power components, such as the pump, to permit pumping of the production fluids to the surface or some point above the surface of the earth. A typical submersible motor, used to pump production fluids from beneath the earth""s surface, has an outer housing substantially sealed from the production fluid environment and sized to fit within standard wellbore casings. An exemplary submersible motor is a three-phase induction-type motor, having a shaft rotatably mounted within the housing such that it is in general alignment with the axis of the wellbore when residing in the wellbore.
Power may be supplied to the submersible electric motor via a power cable that runs along the deployment system. Typically, the power cable is banded or supported along either the outside or the inside of the deployment system. Generally, the power cable is routed to the electric motor to supply electric power thereto, and the submersible pump is powered by the motor by way of an appropriate drive shaft.
A variable speed drive may be used to control the speed at which the motor operates and to facilitate an overall increase in efficiency of operation. However, variable speed drives can create undesirable harmonics. For example, certain harmonics created by the operation of the motor cause problems such as voltage distortion, zero-crossing noise, overheating of transformers, nuisance tripping of circuit breakers, and over-stressing of power factor correction capacitors. The lower harmonics are particularly troublesome because they are larger, therefore less attenuated by the system impedance and more difficult to remove at the source. Additional losses arise due to the production of harmonic generated fields. Furthermore, harmonics, whether propagated or not, cause increased losses in the transformer windings and core. Presently, separate components such as filters and inverters are used to reduce the amount of harmonics in the signal output by the variable speed drive. Although, this solution produces some results, the filter components are separate from the variable speed drive. Additionally, there are still harmonics produced that are not filtered out by these components. These harmonics remain to produce the aforementioned undesirable affects.
Certain aspects commensurate in scope with the originally claimed invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below.
An aspect of the present technique provides a variable speed drive output with reduced harmonic components. In this embodiment, certain components of the variable speed drive are integrated so that a sine wave output is produced with a reduced amount of harmonic components. More particularly, certain variable speed drive components that have filtering characteristics may be utilized to filter out harmonic components in a sine wave output. Thus, an electric motor can be operated efficiently without the detrimental introduction of harmonics into the system.
In another aspect of the present technique, a variable speed drive having a plurality of three phase inverters are integrated with inductors and a sine wave filter to produce a sine wave output having reduced harmonic components. The inverters and inductors are integrated in a way so that they perform functions similar to the sine wave filter. Consequently, the use of the integrated variable speed drive features along with the sine wave filter facilitates the production of a cleaner sine wave output.